Looking for exotic matter

Late last year, researchers thought that they had discovered the first example …

One of the little-known aspects of physics' standard model is that there is a whole lot of it that remains untested. We have leptons, consisting of electrons, muons, and tauons, and we have hadrons, which are made up of combinations of quarks. All hadronic matter that we have observed is either baryonic in nature, constructed from three quarks, or are mesons, which have two quarks. But nature has all these quarks just laying around—it seems a waste not to lump more of them together, right?

Well, theoretically, many of these combinations of quarks are thought to be stable, but, until late last year, no one had observed any of these exotic hadrons. Late last year, the Belle collaboration reported the discovery of a particle, called Z�(4430), which consists of four quarks. Now that theorists have some real data to sink their teeth into, they are trying to figure out how such an exotic hadron is structured.

The new hadron can be though of as two closely bound mesons, or a purely four-quark construction, or even something called baryonium—but which is it? From the decay data, the researchers know that a double meson will consist of charm/anti-up and anti-charm/down. On the other hand, a four-quark construction will result in meson-like pairs of charm/up and anti-charm/anti-down, while the elusive baryonium is more complicated, consisting of coupled baryons (three quarks): charm/up/unknown and anti-charm/anti-down/anti-unknown.

To distinguish among these possibilities, a group of researchers from Ohio State University have tried to calculate how tightly each configuration is bound and what this means for the various ways in which Z�(4430) reveals itself. The whole point of these calculations is that, as new collisional data comes in, researchers will be able to distinguish the internal structure of the Z�(4430) by observing which decay products are most prevalent. They can then use these findings to predict the properties of other exotic hadrons.

What happens next in the paper is one of the things that excites me most about science. The researchers note at the very end of the paper—a note that was obviously added after the paper was accepted for publication—that a competing collaboration (BABAR) had failed to find the signature decay components of Z�(4430). This may cast doubt upon the apparent discovery of Z�(4430) by the Belle collaboration, or it may be that they got lucky—other decay signatures may be more likely, and BABAR was looking in the wrong place. All I know is that it is a bit strange.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.